A roller type speed reducer is disclosed Japanese Unexamined Patent Application Publication No. S62-93565. This roller type speed reducer includes an input shaft and an output shaft coaxially arranged such that axial ends of the input and output shafts face each other. An internal gear having on its inner periphery a plurality of internal teeth having a curved shape is supported by housings covering the axial ends of the input and output shafts. The input shaft is provided at its axial end with two eccentric disks rotatable inside the internal gear and spaced axially from each other. The output shaft is provided at its axial end with a cage arranged between the internal gear and bearings press-fitted to the outer diameter surfaces of the eccentric disks. The cage is formed with a plurality of circumferentially equidistantly spaced apart pockets, the number of which is smaller than that of the internal teeth, to face the respective eccentric disks. Rolling elements in the form of rollers are received in the respective pockets, the rolling elements being configured to roll along the outer diameter surfaces of the bearings and to mesh with the internal teeth of the internal gear successively.
In the speed reducer configured as described above, the rolling elements mesh with the internal teeth of the internal gear successively when the input shaft and thus the eccentric disks rotate. The rolling elements are moved in the circumferential direction by the distance equal to the width of one tooth of the internal teeth per rotation of the input shaft, so that the output shaft rotates at a reduced speed relative to the input shaft.
In order to smoothly transmit rotation, in the speed reducer described in Japanese Unexamined Patent Application Publication No. S62-93565, the tooth shape of each internal tooth is identical to the (curved) trajectory of the outermost portion of one rolling element, which is parallel to the trajectory of the center of the rolling element, when the output shaft rotates within one pitch of the internal teeth of the internal gear, and the eccentric disks rotate correspondingly such that all of the rolling elements contact the internal teeth.
The conventional roller type speed reducer described above is generally used to reduce the rotation speed of an electric motor connected to the speed reducer. In conventional motor assemblies including the above-described motor and speed reducer, since the rotating shaft of the electric motor is connected to an input shaft of the roller type speed reducer by a shaft coupling, the motor assembly tends to be large in size. As a result thereof, it is necessary to secure large space for mounting such a motor assembly in various devices.
Furthermore, in such conventional roller type speed reducers, the input shaft is integrally formed with eccentric disks by which rolling elements mesh with internal teeth of an internal gear successively, so that the eccentric disks rotate together with the input shaft at all times. Therefore, if excessive torque is applied to the input shaft suddenly, the rolling elements may get stuck and locked between the tooth surfaces of the internal teeth and the outer diameter surfaces of bearings press-fitted to the outer diameter surfaces of the respective eccentric disks. In light thereof, conventional roller type speed reducers need to be improved in order to enhance reliability.